James Murphy ought to be unwinding for Christmas after a golden year for his advertising agency adam&eveDDB and its most celebrated client, John Lewis, but he is not resting on his laurels. “Success for us tends to fuel angst — rather than complacency,” says Murphy, who adds half-joking: “Only the paranoid survive.”

Yet things couldn’t be going much better for London’s hottest ad agency, which Murphy co-founded in 2008 and merged with Omnicom’s venerable DDB in 2012. It was named the best agency in the world at advertising’s Oscars, the Cannes Lions, in June. Revenues are up 20% year on year as it has won 10 out of 11 pitches for new clients including Virgin Atlantic and Haig Club whisky. And now it has produced its biggest Christmas hit yet for John Lewis, with its Monty the Penguin ad.

The TV commercial, featuring a boy who dreams about what to get his toy penguin for Christmas, has had almost 20 million views on YouTube and spawned endless spoofs. Monty, complete with cuddly toy range, has also helped the department store chain’s weekly sales to smash £100 million at the earliest stage in the year on record — proof that advertising works.

But Murphy, who grew up near Norwich and shopped at John Lewis with his family as a child, is already thinking about how his team can top this year’s effort. “We will be briefed by John Lewis about the 2015 Christmas campaign in about a week’s time,” he explains, revealing just how far ahead they have to plan.

Paddington-based adam&eveDDB has had success for four Christmases in a row with John Lewis after The Bear And The Hare (2013), The Snowman and Snow-woman (2012) and the boy who couldn’t wait to give a gift to his parents (2011). All had a similar theme — understated but emotional stories, a great soundtrack, and a focus on the virtue of giving — but even Murphy admits his team has been “pretty stunned” by the reaction to the Monty ad. “It has been a step change compared with other years, which we didn’t think was possible.”

adam&eve/DDB has practically become the Christmas agency as it has also done festive campaigns for Mulberry, Harvey Nichols and Unilever’s mustard brand Maille.

Although many of its clients are quintessentially British — they include the Daily Telegraph and the Financial Times — more than half of this year’s 20% increase in revenues has come from international clients moving work into London from Europe. German giant Volkswagen’s commercial vehicles and Maille from France are examples.

Murphy says this flood of work shows London’s strength as a global creative hub. It matters as international brands increasingly want one “master hub” country instead of lots of “boots on the ground” in every key market. The time zone between Asia and America and the city’s cosmopolitan mix and “ultra-competitive” spirit all help, as the UK ad industry enjoys a resurgence not seen since the early Eighties, he says.

But Murphy feels strongly that the rise of the Scottish nationalists and Ukip are a double threat — so much so that adam&eveDDB worked on a social media campaign, called Heartstrings, ahead of September’s independence vote. Britons living outside Scotland were encouraged to go online and post a reason why they love Scotland being part of the UK.

“We aren’t unanimous in our party political affiliations,” he says of the adam&eveDDB management team. “But we certainly all felt the European question and the Scottish question are linked. We felt on both fronts that the risk is we diminish ourselves as a country and as an economy — that it would be a retrograde step.”

Murphy feels especially strongly about Ukip’s demand to leave the European Union. “It seems very risky and very sad that we could turn our backs on a situation where the UK is world-class on so many fronts — especially in marketing services and financial services. The idea that we should shut ourselves off from our European partners and cut out ourselves off from that investment and those clients seems a shame. This idea of withdrawal is a rejection of something, not an embrace of something.” Global clients “would be baffled if we were to shoot ourselves in the foot”.

Murphy didn’t plan a career in the ad industry. After university in Essex, he worked in London as a delivery van driver for a year when someone suggested he might like advertising. “I found out where the main agencies were and whenever I had a delivery, I popped into the reception and asked, ‘Do you have a job in the mail room?’ ” He ended up with just that for an agency called Bates before landing a trainee role at Ogilvy & Mather. Then he moved to Rainey Kelly Campbell Roalfe, a hot start-up that advertising giant WPP ended up buying and merging with its Y&R agency. Murphy became chief executive aged 36, overseeing premium brands such as Marks & Spencer and Lloyds Banking Group.

But he and fellow Rainey Kelley executives David Golding and Ben Priest were restless and wanted the freedom of starting up on their own, which they did with another partner, Jon Forsyth, in 2008.

Despite the recession, adam&eve quickly grew with its open, collaborative approach and a policy of “no hierarchies, no departments”. Murphy says: “You want to encourage this creative energy and chaos so the best ideas grow.”

America’s Omnicom came calling in 2012, wanting to revitalise DDB in a similar way to how Y&R turned to Rainey Kelly. Murphy and his partners were happy to sell for an estimated £60 million, if they hit targets. Last year, it made £9.5 million profit on sales of £51 million. Nearly half their work is in digital and social, with content booming.

One of their first acts after moving into DDB’s Paddington office was to knock down the warren of small offices and internal walls to create an open-plan space in the former postal sorting office. From five staff in 2008, it now has 580.

Murphy is cheerful and easy-going yet disciplined. When the agency wins a lot of work, it shuts its doors to new business for months. “If you were to plot our growth, it would look like a staircase. You win business. Then you have to stop for a while and consolidate. You sometimes get agencies who are very good at winning new business but not so good at keeping old business.”

It feels like a fun place to work, with a “Hot Man” wall, dreamed up by two female Welsh creatives, which is covered in male pin-ups cut out from magazines and newspapers. There is no Hot Woman wall, Murphy notes drily, and they work too hard to have a Mad Men-style drinks cabinet full of booze.

He is married to an advertising executive and is the father of two daughters, and that spirit pervades the agency. “There’s a family thing with the co-founders who respect each other and consult each other,” he says, and he sees a common theme with their work for John Lewis: “It’s about the journeys that families go on together.”